It is known that acrylate containing coating materials can be cured either thermally or by radiation in the presence of a free radical photoinitiator. However, it is well recognized that thermal curing is not cost efficient and that radiation curing in free radical systems is oxygen inhibited, usually requiring an inert atmosphere or the addition of a hydrogen donating component. It has been found that polymerization or curing in free radical systems ceases almost immediately upon removal from the source of radiation; thus, the cured product often contains significant amounts of unpolymerized components in a discontinuous protective coating. Although epoxides and vinyl ethers are known to cure by cationic polymerization in the presence of onium salt initiators, these initiators, being salts are difficult to solubilize and thus the choice of suitable reactive diluents is greatly restricted. The choice of suitable reactive diluent may be increased by incorporation of appropriate solvents into the coating formulation. However the addition of non-reactive diluents with low volatility in the formulation adversely affect the coating properties. Accordingly, it is an aim of research to develop monomers which provide stable formulations with the above polymerizable materials and which solubilize the cationic initiator in order to promote radiation induced cationic polymerization and to incorporate the beneficial properties of the vinyl ethers, epoxides, acrylates or urethanes into the finished product. Additionally it is desirable that such monomers or their oligomers be amenable to radiation curing at a rapid rate under mild temperature conditions in an onium salt initiated polymerization which is not oxygen inhibited and which permits continued polymerization after removal from the source of radiation exposure. It is further desired that the reactive monomer be capable of minimizing undesirable properties of certain coating materials, such as the acrylate monomers which are known skin irritants and that it also serve as a solvent for the initiator.
Accordingly, it is an object of this invention to provide a reactive diluent for cationically induced radiation polymerizations of polymerizable monomers and oligomers normally not polymerized in cationic systems due to their incompatibility with onium salt photoinitiators.
Another object of this invention is to provide a solvent for an onium salt photoinitiator which is economically and conveniently prepared from commercially available compounds.
Another object is to provide a mono-functional reactive diluent capable of modifying cross-linked density and flexibility of the resulting polymerized coatings.
Another object is to provide an onium salt photoinitiator diluent which is incorporated into the cured product of a radiation curable resin.
Still other objects are to provide a process for coating a substrate with an improved protective coating and to provide a substrate which is highly resistant to abrasion and chemical attack.
These and other objects of this invention will become apparent from the following description and disclosure.